This invention relates to radio communication systems. In particular, it relates to ad-hoc radio systems that use frequency hopping in cordless telephony scenarios, such as cellular radio telephony and wireless local area networks (WLANs).
In the last decades, progress in radio and integrated circuits using very large scale integration (VLSI) technology has fostered widespread use of radio communications in consumer applications. Portable communication devices, such as mobile telephones, can now be produced having widely acceptable cost, size, and power consumption. Computing devices have experienced the same progress, leading from the desktop personal computer (PC) to notebook computers to sub-notebook computers and to personal digital assistants (PDAs). Today""s miniaturized radio systems enable such computing devices to communicate wirelessly, either by themselves or in combination with mobile radio telephones. The separation between the telecommunication industry on the one hand and the PC industry on the other hand is gradually vanishing.
Many commercial wireless communication systems use the radio spectrum ranging from about 800 MHz up to about 5000 MHz (5 GHz). The radio spectrum is a scarce resource, and with the increasing need for that resource due to both the demand for increased bandwidth (higher data rates) and the continuously rising number of users, efficient spectrum usage is essential if the increasing need is to be met. This holds both for the spectrum controlled by communication system operators (licensed bands used for example for public radio telephony) and the spectrum that is free for all to use (unlicensed bands like the globally available industrial, scientific, and medical (ISM) bands at 900 MHz, 2400 MHz, and 5700 MHz).
Most radio systems in consumer applications today, like cellular or cordless telephony, wireless Internet, etc., consist of a fixed, wired infrastructure and portable terminals. The portable devices reach the infrastructure via access points or base stations. To be spectrally efficient, the radio spectrum is re-used in different geographical locations of the infrastructure. This means that independent users can use the same radio channel, provided their geographical separation is such that their signal-to-interference ratios, where the interference is co-channel interference, are good enough for acceptable reception. The current kind of interaction between wireless remote terminals and fixed infrastructures is acceptable since most communication takes place between wireless terminals and devices wired to the infrastructure (like a desk telephone, a computer server, a PC connected via a cable modem, etc.). However, wireless user scenarios are changing, and with the increasing proliferation of cheap radio transceivers, communication between portable devices will continue to increase. In particular, local (short-range) communication sessions between devices like laptop computers, printers, mobile phones etc., will increase in the coming years.
The current architecture of a wireless system like a cellular radio telephone system has a fixed part comprising a plurality of geographically separated base stations and a number of remote terminals, many of which may be portable. FIG. 1 depicts the typical architecture of a radio system 100, comprising fixed base stations (BS) 110, which have respective radio coverage areas (cells) 120 indicated by the dashed lines, and portable terminals 140. Each BS 110 typically has one or more radio transceivers that provide interfaces between the public switched telephone network and portable radio telephones and other remote terminals located in its cell. The fixed site transceivers 110 and remote terminals 140 communicate by exchanging RF signals, employing various formats (analog, digital, and hybrids) and access techniques (frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrids). General aspects of such cellular radio telephone systems are known in the art.
Each BS 110 handles a plurality of traffic channels, which may carry voice, facsimile, video, and other information, through a traffic channel transceiver that is controlled by a control and processing unit. Also, each BS includes a control channel transceiver, which may be capable of handling more than one control channel and is also controlled by the control and processing unit. The control channel transceiver broadcasts control information over the control channel of the BS or cell to terminals 140 that are xe2x80x9clockedxe2x80x9d, or synchronized to that control channel. This locking allows the terminals 140 to be reached quickly and to reduce their power consumption since they are synchronized to the base stations.
In an FDMA radio telephone system, each remote terminal is allocated a radio channel (e.g., an RF carrier signal for transmitting and an RF carrier signal for receiving) for the duration of a communication session with the fixed site transceiver(s). The transmitters in the remote terminal and in the fixed site typically produce RF power levels that are controlled in a way that minimizes interference with other transmitters in the system.
In a TDMA radio telephone system like the Global System for Mobile communications (GSM) currently in use in many parts of the world, each carrier signal is time-shared by up to eight radio telephones, i.e., each carrier signal transports successive frames of eight time slots each. During its assigned time slot, a radio telephone tunes its transmitter to the proper frequency, ramps to the output power level, transmits the desired information, and then ramps down so as not to interfere with other users. In a GSM system, the length of each time slot is about 577 microseconds.
In a typical direct sequence CDMA (DS-CDMA) system, an information bit stream to be transmitted is superimposed on a much-higher-rate bit stream that typically consists of consecutive symbols that are sometimes called spreading sequences. Usually, each information bit stream is allocated a unique spreading sequence that is consecutively repeated to form the much-higher-rate bit stream. Each bit of the information bit stream and the spreading sequence are typically combined by multiplication, or modulo-2 addition, in a process sometimes called coding or spreading the information signal. The combined bits stream may be scrambled by multiplication by another, usually pseudo-noise, bit stream, with the result transmitted as a modulation of a carrier wave. A receiver demodulates the modulated carrier and correlates the resulting signal with the scrambling bit stream and the unique spreading sequence to recover the information bit stream that was transmitted.
In North America, a digital cellular radiotelephone system using TDMA is called the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the TIA/EIA/IS-136 standard published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA). Another digital communication system using direct-sequence CDMA is specified by the TIA/EIA/IS-95 standard, and a frequency-hopping CDMA communication system is specified by the EIA SP 3389 standard (PCS 1900). The PCS 1900 standard is an implementation of GSM for personal communication services (PCS) systems. In these communication systems, communication channels are implemented by frequency-modulating RF carrier signals that have frequencies near 800 megahertz (MHz), 900 MHz, 1800 MHz, and/or 1900 MHz. One form of GSM radio telephone communicates with an associated base station in the frequency bands 890-915 MHz (up link) and 935-960 MHz (down link), with each frequency band being divided into 124 channels with a separation of 200 KHz.
Communication between portable terminals is possible in these systems, of course, but today this is always mediated by one or more base stations 110. For example, if both terminals are locked to the same base station, that base station establishes a first radio link to one of the remote terminals and then that base station establishes a second radio link to the other remote terminal. After the links are established, i.e., during the communication session, the base station relays information received from one terminal to the other and vice versa. FIG. 2 illustrates this exemplary situation of a terminal-to-terminal link between remote terminals 240, 250 that is mediated by a single base station 210. Of course, it will be appreciated that one base station can pass information to another base station as part of the relay between the terminals.
This is not an efficient use of the radio spectrum when the terminals are close enough together that they could communicate directly since two links (radio channels) have to be established (three radio units are involved) in the session when only a single link or channel would suffice. The system""s capacity is thus reduced by a factor of two. A common scenario for this kind of operation is the intercom feature of a cordless telephone. In this application, the terms xe2x80x9cwirelessxe2x80x9d and xe2x80x9ccordlessxe2x80x9d will be used interchangeably, although xe2x80x9ccordlessxe2x80x9d is sometimes thought to refer only to short-range communication. To improve efficiency, terminals should communicate with other terminals directly if possible, forming direct xe2x80x9cad hocxe2x80x9d connections, i.e., connections that do not involve relaying by a base station all of the information to be exchanged. Wireless systems that support ad hoc connections should provide direct communication among any radio units that are in range of one another.
Such local, ad hoc sessions can use the ISM bands at 900 MHz and 2400 MHz, which have been opened for commercial applications, and now many products have been introduced providing wireless communications in these bands. The use of these radio bands is restricted in the United States by Part 15 of the rules of the Federal Communications Commission (FCC) and in Europe by ETS 300 328 of the European Telecommunication Standards Institute (ETSI). Other countries apply similar rules. Briefly stated, the rules require a user to spread its transmitted power over the band in order to minimize the chance of interference with other users.
Such spreading can be achieved by either of two techniques that are well known in the art of spread spectrum communication systems, which include systems using the CDMA technique described above. In the frequency hopping (FH) technique, the transmitter emits a carrier signal that is modulated in a conventional way by the information to be sent and the frequency of the carrier signal is changed (hopped) according to a predetermined pattern that is known to the receiver. GSM-type systems are well-known examples of FH systems. In the general direct-sequence (DS) technique, binary information to be sent is combined with a noise-like, higher-bit-rate binary sequence that is known to the receiver and the combination sequence modulates a carrier signal having a fixed frequency.
Both spreading techniques have merits and drawbacks, but with the current state-of-the-art, FH spreading results in less complex and cheaper radio transceivers than DS spreading. FH systems operating in the U.S. in the 2.4 GHz band (2400 MHz to 2483.5 MHz) are required to hop among at least seventy-five frequencies. A new relatively low cost, low range wireless transmission system (defined by the recently developed xe2x80x9cBluetoothxe2x80x9d technology) has been proposed for localized two-way data transmissions which is based on the FH concept. Bluetooth systems are envisioned as a universal radio interface in the 2.45 GHz frequency band that enables portable devices to connect and communicate wirelessly via short-range, ad-hoc networks. Readers interested in various details regarding the Bluetooth technology are referred to the article entitled xe2x80x9cBLUETOOTHxe2x80x94The universal radio interface for ad-hoc, wireless connectivityxe2x80x9d authored by Jaap Haartsen and found in Ericsson Review, Telecommunications Technology Journal No. 3, 1998, the disclosure of which is incorporated here by reference.
For a Bluetooth compatible system, the 2.4 GHz ISM band is divided into seventy-nine channels, starting at 2402 MHz and spaced apart by 1 MHz as illustrated by FIG. 3. Two or more radio units communicate by hopping their carrier signals from one channel to another channel, e.g., from 2405 MHz to 2480 MHz to 2463 MHz, etc., according to a pseudo-random pattern. As long as the units hop in synchrony and with the same phase, they simultaneously use the same channel and thus stay in contact with each other.
FIG. 4 is a general diagram of a radio transceiver 400 that can operate according to the Bluetooth system and that has a transmitting (TX) section 402 and a receiving (RX) section 404. Information to be transmitted is passed by an input of the TX section 402 to an encoding device 406, which processes the input information according to an error-correcting code and, perhaps, a security code. Forward error correction and scrambling codes are well known in the art of cellular radio telephony, so they need not be described in more detail here. The encoded information produced by the encoder 406 is provided to a modulator/up-convertor 408, which modulates a higher-frequency carrier signal with the encoded information according to the format and access technique employed by the system. The magnitude of the modulated carrier produced by the modulator/up-convertor 408 is then typically increased by a power amplifier (PA) 410, which provides its output signal to a suitable TX antenna 412. In the RX section 402, a portion of a modulated carrier signal received by a suitable RX antenna 414 is provided to a low-noise amplifier (LNA) 416, which increases the magnitude of the received portion without adding substantial noise and provides the increased modulated carrier signal to a down-converter 418. The carrier signal is stripped away by the down-converter 418, and the resulting signal is provided to a demodulator 420 that in essence produces a replica of the encoded information that modulated the carrier signal. That encoded information is decoded by a decoding device 422, and the received information is passed by an output of the RX section 404 to a user or further processing devices. The modulator/up-convertor 408, down-convertor 418, and demodulator 420 carry out their functions using local oscillator and other signals having precisely controlled frequencies that are generated by a suitable synthesizer 424. The frequencies selected for the synthesizer and other aspects of the transceiver 400 are determined by control signals provided by a controller 426.
In a FH system that can be used for setting up ad hoc connections, it is necessary to ensure that the units hop synchronously and in phase in order for communication to be maintained: the TX hop of one unit must be the RX hop of the other unit or units, and vice versa. The hopping scenario may be determined by the identity and system clock of one of the communicating devices (which is designated as the master), as is done in the Bluetooth system. That is to say, the hopping sequence is determined by the identity of the master unit, and the phase in the hopping sequence is determined by the system clock of the master unit. All other devices (slaves) that are to participate in the communication session must use the same master identity and synchronize to the system clock of the master unit in order to be synchronized to and in phase with the FH sequence. In principle, any unit can be designated as a master. The system clocks in the units are typically free-running; it is only during a communication session that respective clock offsets are temporarily added in order to adjust the slave devices"" clocks to the master device""s clock.
Purely ad hoc communication systems have certain disadvantages. Radio channel allocation is not coordinated between the terminals, with the result that the terminals have to put a lot of effort into finding one another. A terminal must find a channel that is acceptable with respect to users of surrounding communication systems as well as the terminal it is calling and other users of its system. Adaptive channel selection schemes can be implemented that avoid collisions between users seeking to transmit on the same channel and other systems, but these usually put a large burden on the terminals with respect to power consumption, complexity, and costs and are not suitable for ad hoc communication systems with distributed control.
A larger number of channels in a FH radio system is usually preferred because it increases interference immunity. However, increasing the number of channels poses a problem for connection setup. A larger number of channels makes the channel location of a setup or page signal less certain from the point of view of an intended receiver, with the result that either the receiver must increase its amount of channel scanning (so its power consumption increases) or the delay before a connection is established increases. Using a limited number of dedicated control channels can reduce the uncertainty, but often this is either not allowed (as, for example, in the unlicensed ISM band) or not practical (due to channel re-use requirements).
The trade-off between idle mode power consumption and access delay is a serious problem with ad hoc communication systems. Radio units in a truly ad hoc system are not locked to any other device until a communication channel has to be established. When a unit is idle (i.e., not participating in a communication session), the unit must listen for other units that want to contact it. Depending on the system""s degrees of freedom (i.e., the number of frequencies, time slots, or codes used), it may take some time before contact can be established, especially when an idle unit only xe2x80x9cwakes upxe2x80x9d and scans for incoming signals very seldom in order to save power. To reduce the setup time of a communication session, an idle unit usually has to wake up and scan more often, which increases the unit""s power consumption. Since power consumption is usually an issue (especially in portable devices), scanning usually must be minimized, which can result in unacceptable delay during connection setup.
Typically, a FH radio unit in standby or idle sleeps most of the time, but periodically listens to a selected hop channel for a page message. To obtain immunity against interferers, a different hop channel is selected each time the radio unit wakes up, the hop sequence followed is based on the unit""s identity, and the phase is determined by the unit""s free-running clock. A FH radio unit that wants to page another FH radio unit in order to set up a connection does not know when the other radio unit will wake up or which channel the other radio unit will be tuned to, so the paging radio unit must send its page message repeatedly on many hop channels. There are many ways to do this, some of which are described in allowed U.S. patent application Ser. No. 08/771,692 filed on Dec. 23, 1996, by J.C. Haartsen and P. W. Dent for xe2x80x9cAccess Technique of Channel Hopping Communications Systemxe2x80x9d that is expressly incorporated here by reference, and in U.S. Pat. No. 5,353,341 to Gillis, U.S. Pat. No. 5,430,775 to Fulghum et al., and U.S. Pat. No. 5,528,623 to Foster, Jr. Other ways can for example be found in the IEEE 802.11 WLAN standard. The set-up time or access delay (the time its takes for two radio units to synchronize their frequency hopping) and the effort to get two units hop synchronized increases when the number of hop channels increases and/or the ratio of time spent listening to time spent sleeping decreases.
For cordless telephone applications, the setup time is a critical parameter; it may take too long to establish an ad-hoc connection. To decrease the ad-hoc setup time, either the uncertainty in frequency and/or the uncertainty in time must be reduced. Reducing the frequency uncertainty means reducing the number of channels (frequencies) that must be searched for a page message; this reduces the system""s immunity against interferers and may violate the rules for using the spectrum, even an unlicensed band like the ISM band. Reducing the time uncertainty means increasing the ratio of time spent listening to time spent sleeping, but this results in more power consumption, which is undesirable for many devices, like portable telephones.
One solution is to lock the portables (remote terminals) to a FH base station even before a connection is established. If a terminal comes in range of the base station, it automatically locks to that base station and can then go to sleep, i.e., enter a low-power mode, as described for example U.S. patent application Ser. No. 09/210,594 filed on Dec. 15, 1998, by J.C. Haartsen and J. Elg for xe2x80x9cCentral Multiple Access Control for Frequency Hopping Radio Networksxe2x80x9d, which is expressly incorporated here by reference. In the low-power mode, the remote terminal wakes up periodically and listens to a broadcast or beacon message transmitted by the base station. Since the terminal and base station are locked, i.e., synchronized, the ratio of the time spent listening to the time spent sleeping can be very small since the terminal knows when and where to listen for the beacon. Therefore, when locked, the terminal can enter a low-power mode having very little scanning activity and can still make and receive calls with a reasonably fast access time. Locking a terminal to a beacon channel in this way is adequate when connections are made between the terminal and the PSTN via a base station, but not for intercom calls in which one remote terminal wants to connect to another remote terminal.
It is therefore an object of this invention to increase spectral efficiency while reducing setup time and power consumption in cordless telephony systems employing FH, and to provide efficient spectrum utilization for ad-hoc and intercom connections while still obtaining fast connection setup with low power consumption in FH cordless systems.
Applicants"" invention solves the problems of previous ad hoc FH communication systems and provides a wireless communication system having a base station that acts as a master, transmitting a beacon channel to which all sufficiently nearby remote terminals are locked. The master base station""s identity and system clock determine the hop sequence and hop sequence phase, respectively. In an intercom call (ad-hoc connection) between two remote terminals, one terminal acts as a master, establishing its own hop sequence and phase based on the terminal""s identity and system clock. In order to enable a fast setup of the connection between the terminals, the identity and clock of the master terminal is relayed by the master base station to the other terminal or terminals. Upon completing this information exchange via the base station, the base station orders the terminals to make a connection in an otherwise conventional ad-hoc way. Since the terminals have information regarding the hop sequence and phase to be used, there are substantially no uncertainties in either frequency or time and thus the ad-hoc connection between the terminals is established very quickly.
A central radio base station controls the traffic both for base-to-terminal connections and for terminal-to-terminal connections. The central base station can be a conventional base station in a cellular system, an access point in a WLAN system, or a cordless base station in a cordless telephone system. All remote terminals within range of the central base station are locked, i.e., synchronized, to the central base station by, for example, listening to a known broadcast channel or another control channel or beacon. While locked, a terminal can idle in a very low power consumption mode since the terminal needs only to wake up at regular intervals to listen to the known broadcast channel.
The central base station preferably has all the intelligence needed to determine what channels are attractive to use. This determination can be based on either a fixed channel plan, like the channel plans known in cellular radio telephone systems, or adaptive channel allocation measurements, or a random channel selection. In addition, the central base station knows all of the terminals"" characteristics, like timing and sleep behavior. When a terminal wants to communicate with another terminal, the initiating terminal sends a request for a setup to the base station it is currently locked to. This base station then informs the terminal to be connected of this request, and this base station informs both terminals of the channel (and perhaps the timing) to use for the connection. The terminals then switch to the new channel and establish a connection.
The two terminals preferably remain locked to the central base station, viz., they continue listening to the known broadcast channel, enabling the base station to keep control over the link between the terminals. For example, if the terminal-to-terminal channel deteriorates, the terminals may request assignment of another channel. Also, maintaining the lock can prevent the terminals from (both) moving into an area where their terminal-to-terminal connection may cause interference to others, such as the users of a conventional wireless system. The terminals may advantageously report their own measurements of the radio spectrum to the central base station, thereby assisting in channel selection.
In one aspect, the present invention provides a method for establishing an ad hoc communication session between remote communication terminals in a FH communication system. The method comprises the steps of locking the remote terminals to a base station, establishing a communication link between a first remote communication terminal and the base station, and establishing a communication session between the first remote communication terminal and the second remote communication terminal using information retrieved from the base station.
In another aspect, the invention provides a system for establishing an ad hoc communication session between remote communication terminals. The system comprises a base station adapted to transmit a beacon signal over a broadcast channel, wherein the beacon signal includes information identifying the base station and information indicating base station""s clock value. The system further includes at least a first remote communication terminal adapted to receive the beacon signal and to establish a communication link with the base station when the first remote communication terminal receives an instruction to establish a communication session with a second remote communication terminal. The base station establishes a communication link with the second remote communication terminal and transmits to the first and second remote communication terminals information and instructions causing the first communication terminal to establish a direct communication link with the second remote communication terminal.